What a great combo! I bought the 870 Express with an 18" barrel at a gunshow recently for $200 and then got the Knoxx set on ebay for $75. What a deal!
I recently shot it next to a H&R Topper 20 gauge and the 870 had less recoil. I am totally satisfied with the 870 and it will be my HD weapon from now on.
I put a side saddle on it for an additional 6 rounds and added a fiber optic sight so the whole package cost less than $340.